Mixing and kneading aluminum powder with a synthetic resin is a known method for coloring a synthetic resin to give a metallic finish. Aluminum particles having a plate-like shape or a spherical shape are commonly known, and plate-like particles can give high brightness with a glittering granular appearance because the flat and smooth surface of these particles uniformly reflect light.
Use of the aluminum powder by itself is associated with a risk of dust explosion. Thus, the aluminum powder is usually wetted by a solvent such as mineral turpentine before use.
In the case where such aluminum powder that has been wetted by a solvent is mixed and kneaded with a synthetic resin, it causes the solvent to remain in the synthetic resin. Thus, the residual solvent will be volatilized into a gas during formation of a synthetic resin molded article using such a synthetic resin. This may impair the strength or appearance of the synthetic resin molded article. The residual solvent in the synthetic resin can be removed by heating during production of the synthetic resin molded article; however, it has been difficult to completely remove the solvent by this method.
In addition, the aluminum particles have poor affinity for the resin. Thus, despite attempts to disperse the aluminum particles in the resin, the aluminium particles cannot be sufficiently dispersed, resulting in color unevenness or agglomerations of the aluminum particles.
In order to solve these problems, there has been proposed a method for coloring a synthetic resin using masterbatches substantially free of a solvent which are obtained by solidifying aluminum powder with wax.
As examples of such masterbatches, Patent Literature 1 discloses an aluminum powder-containing resin composition (masterbatch) obtained by mixing an olefin resin, aluminum powder, olefin wax, and quicklime, wherein the aluminum powder content is 40%.
Patent Literature 2 discloses a masterbatch for coloring a synthetic resin, the masterbatch containing an aluminum pigment, polyethylene wax, and a linear low density polyethylene resin, wherein the aluminum pigment is contained in an amount of 60 to 80% by mass, and the polyethylene wax and the linear low density polyethylene resin are contained at a mass ratio of 9:1 to 7:3.